1. Field of the Invention
The invention relates to a process and a facility to remove interfering materials from an aqueous fibrous material suspension with the aid of at least two flotation steps directly or indirectly subsequent to each other.
2. Discussion of Background Information
Processes of the noted type are used to eliminate at least a portion of the undesired solid matter particles suspended in a fibrous material suspension containing fibrous material. It is known that a foam or scum containing the materials to be eliminated is formed in a flotation. A common use of such a process is the preparation of an aqueous fibrous material suspension gleaned from printed recycled paper, in which the printing ink particles are already separated from fibers, so that the ink particles can be floated out. The flotation process described here utilizes the differences between fibrous material and undesired solid matter particles in such a way that the fibrous material remains in the fibrous material suspension, due to its rather hydrophilous character, while the already-mentioned solid matter particles are hydrophobic and reaches the foam with the air bubbles. Alongside the printing ink particles, there are other materials which are hydrophobic and thus can be separated by flotation from the fibrous material. In particular, such materials are adhesives, fine synthetic material particles, and perhaps also resins. Since fibers are separated from contaminations via the flotation process and not all fibrous material parts are sorted out, the term "selective flotation" may be appropriate. The likewise utilized term "flotation de-inking" is generally not only used for the removal of printing ink particles but rather more generally for the selective flotation of contaminations from fibrous material suspensions.
Cleaning procedures with two partial steps, a pre-flotation and a post-flotation and mechanical material treatment in the interim, have had a particularly good effect, such as is 5 described in the subject matter essay "Flotationsdeinking--Grundlagen und Systemeinbindung," H. Britz, published in the Wochenblatt fur Papierfabrikation 10-1983.
The prior art with respect to flotation procedures for fibrous material suspensions has already advanced quite far. Therefore, solutions exist which are certainly suited for removing a large portion of the interfering material particles by flotation. It is disadvantageous, however, when the spectrum of magnitude of the interfering material particles to be removed is very broad. In such cases, problems frequently appear, since the flotation facilities cannot always encompass the whole spectrum of particle size.
It is known that gas pockets will rise in a fluid against a gravity field. This field of gravity is due to the Earth's gravity and can be intensified significantly. In DE 44 26 159 A1, a procedure is described, in which the fibrous material suspension to be cleaned first reaches a flotation cyclone working with centrifugal force and then flotation cells of conventional construction, which, due to the field of gravity, work essentially alone. The flotation foam of the last-named flotation cells--perhaps also of the flotation cyclone--is cleaned in a further flotation cyclone which forms the secondary phase. This process works with the combination of different flotation steps, but it is not sufficient in some cases to produce a completely white de-inking material.